Things to know: Mayweather-Marquez

1. Floyd Mayweather returns to the ring following a 651-day layoff (by far the longest gap between fights in his career).

In his last outing, Mayweather handed Ricky Hatton his first career loss via 10-round TKO on Dec. 8, 2007.

2. The fight will be contested at a catchweight 144-pound limit. Marquez will be contesting a bout above the 135-pound lightweight limit for the first time in his career and how he carries the added weight is a major storyline in this showdown. For comparison's sake, Marquez's rival Manny Pacquiao carried the added weight very well in stopping Oscar De La Hoya in December.

3. Marquez is the No. 2 pound-for-pound fighter in the world according to The Ring magazine and is also Ring's lightweight world champion. Mayweather was rated the top pound-for-pound fighter by the same publication from July 2005 to June 2008.

4. Including The Ring magazine belts, Mayweather is one of five fighters to have held major world titles in five different weight divisions (he is joined by Oscar De La Hoya, Manny Pacquiao, Sugar Ray Leonard and Thomas Hearns).

5. When he retired, Mayweather became the fourth boxer to retire as an undefeated world champion. When he steps back into the ring on Sept. 19, that list will shrink back to three: Rocky Marciano, Joe Calzaghe and Sven Ottke.

6. If Mayweather wins this bout and Pacquiao defeats Miguel Cotto in November, a Mayweather-Pacquiao fight remains a possibility in 2010.